In the demodulation of the Global System for Mobile Communications (GSM), synchronization track breaks down into frequency track and timeslot header position track. The coarse synchronization of the frequency header position uses Frequency Burst (FB), and the coarse synchronization of the timeslot header position uses Synchronization Burst (SB). When the frequency is locked and the synchronization position is searched out, the mentioned two types of coarse synchronization stop. In order to track the multi-path change in the subsequent Normal Burst (NB) demodulation, it is necessary to perform symbol-level synchronization through the training sequence of the NB, measured in timeslots.
FIG. 1 shows synchronization position adjustment in the prior art. As shown in FIG. 1, at the time of adjusting the synchronization position, if the synchronization position of the previous timeslot is a benchmark, channel estimation is performed in the windows of several symbols to the left side of the benchmark and in the windows of several symbols to the right side of the benchmark. The best synchronization position of the current timeslot is found through comparison between the energy value in one path and the energy value in another path, and the received signal of the current timeslot is adjusted. Meanwhile, this synchronization position serves as the benchmark position of the next NB.
FIG. 2 shows a structure of a burst TOA estimation apparatus in the prior art. FIG. 3 shows symbols extracted by a signal extracting module in a burst TOA estimation apparatus in the prior art.
As shown in FIG. 2 and FIG. 3, the existing burst TOA estimation apparatus includes a received signal extracting module and a channel estimating module. The received signal extracting module extracts 28 symbols from the received signals of the current timeslot. The extracted signals are supposed to be Data_I(k),Data_Q(k),k=0, 1, . . . , 27.
The channel estimating module receives a local training sequence. The intermediate 16 bits of the local training sequence are TSC(k) k=0, 1, 2, . . . , 15.
The channel estimating module performs shift correlation for the received signals and the intermediate 16 bits of the local training sequence to obtain 13 complex-valued channel estimates.
            DataEST_I      ⁢              (        k        )              =          (                        ∑                      m            =            0                    15                ⁢                  Data_I          ⁢                      (                          m              +              k                        )                    ×                      TSC            ⁡                          (              m              )                                          )                  k      =      0        ,    1    ,    2    ,    …    ⁢                  ,                  12        ⁢                                  ⁢                  DataEST_Q          ⁢                      (            k            )                              =                                    (                                          ∑                                  m                  =                  0                                15                            ⁢                              Data_Q                ⁢                                  (                                      m                    +                    k                                    )                                ×                                  TSC                  ⁡                                      (                    m                    )                                                                        )                    ⁢                                          ⁢          k                =        0              ,    1    ,    2    ,    …    ⁢                  ,    12  
Because the training sequence that has undergone reverse polarity mapping is ±1, the foregoing algorithm involves only addition.
The energy of the 13 channel estimates is calculated:Energy(k)=DataEST—I(k)2+DataEST—Q(k)2,k=0,1, . . . ,12.
For a single burst, the signals received in the burst are impacted by interference and noise, which may cause deviation of the estimated TOA position. Therefore, a filtering may be performed for the path energy estimate.Energy2(k)=α·Energy2(k)+(1−α)·Energy2−1(k), k=0,1, . . . ,12, where·0<α<1.
The position of the multi-path energy window may vary with the TOA. Because the previous window position is different from the next window position, the definition formula of the a filtering needs to be corrected. For example, this multi-path window position deviates from the previous multi-path window position by one symbol.
For the old path position k, the a filtering is based on:Energyt(k)=α·Energyt(k)+(1−α)·Energyt−1(k+1),k=0,1, . . . ,11.
For the path position k′ generated by the new window, the α filtering is based on:Energyt(k′)=α·Energyt(k′), k′=12.
Afterward, the estimated energy values of the five adjacent channels are added up to obtain nine sums:
            SUM      ⁡              [        i        ]              =                  ∑                  k          =          0                4            ⁢              Energy        ⁡                  (                      k            +            i                    )                      ,      i    =    0    ,  1  ,  …  ⁢          ,  8.
Through the position of the maximum value of the nine sums, the estimated TOA value of the current burst is obtained.
In the process of developing the present invention, the inventor finds that: In the existing TOA estimation, the received signals are used directly for TOA estimation, the signals at the training sequence position of the received signals in the actual radio environment suffer interference (for example, common-frequency or adjacent-frequency interference from a surrounding cell). Therefore, the path energy estimate is not accurate. The TOA estimate obtained based on an incorrect path energy estimate is surely deviated from the correct TOA. The deviation further affects the selection of the search window position in the TOA estimation of the subsequent NB.